The present invention relates to a product and process for making gypsum panels. More particularly, the present invention relates to a gypsum panel having paper with an improved resistance to mold and mildew.
Gypsum panels are well known building products which have been used for years. They are used primarily as interior walls, floor and ceiling products, but also to a certain extent as exterior products. A slurry including calcium sulfate hemihydrate and water is used to form the gypsum core, and the gypsum slurry continuously deposited on a first paper moving beneath a mixer. A second paper is applied over the deposited gypsum slurry and the resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate reacts with sufficient water to convert the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm, thereby forming the gypsum core. The continuous strip formed is conveyed on a belt until the calcined gypsum is set, and the strip is thereafter cut to form panels of desired length. The panels are then conveyed through a drying kiln to remove any excess moisture.
Fungi, such as mold and mildew, may grow in environments where four key elements are present. There must be spores present, nutrients for the fungi to metabolize and water. Temperature is the fourth key parameter for fungus growth, but numerous mold species thrive at the temperatures required for human habitation so this is often considered inherent for mold growth in buildings. Although various environments provide different amounts of each of these elements, water vapor and spores are constantly in the air. The spores require sufficient nutrients to be able to grow if they settle on a substrate where moisture is present.
While various nutrients are generally present in dust particles in the surrounding air, starches also provide sufficient nutrition for mold growth and are often present in both the face paper materials and the gypsum core of gypsum panels. In gypsum panels, starch is frequently used for a number of purposes. Starch may be used as an additive to promote adhesion between the gypsum core and the paper, for example, but cellulosic fibers also provide nutrition for mold growth. Sugar is optionally used to coat particles of calcium sulfate dihydrate that are then used as a set accelerator in the calcined gypsum slurry.
Other starches are also used to modify properties of the gypsum composition. Where gypsum board panels become wet and do not dry out readily, the use of starches in the paper and core materials provides a medium that is suitable for the growth of mold. Gypsum panels, even if not specially treated to make them mold-resistant, still will not usually experience mold growth problems in interior building applications or in other applications if they are kept dry or dry out readily after becoming wet.
However, there are some applications where gypsum panels are desirable for its fire resistance, but where the panels may become wet and not readily dry out. In high-rise buildings, for example, elevator shafts are built before the building is enclosed. One inch thick gypsum panels, such as Sheetrock® brand Gypsum Liner Panels by USG Corp., of Chicago, Ill., are used to line the elevator shafts to provide fire resistance. The elevator shaft wall may be exposed to rain during building construction and may not have the opportunity to dry thoroughly before the building is enclosed. Gypsum panels used in this environment, and other environments where mold growth is possible, are subject to improvement by increasing the resistance of the panels to the growth of mold.
Gypsum panels are known to utilize pressed paper treated with a biocide. Treated paper has generally been less effective to control mold growth for a number of reasons. Many biocides do not retain their efficacy through the process of drying the gypsum panels in a kiln due to the high temperatures and the porosity of the paper. In addition, the water used in the manufacturing of gypsum panels may itself contain mold spores, providing a source of spores from both the air and the set gypsum. Per environmental regulations, there is a limit to the insufficient concentration of biocide that can be present on the surfacing of the paper, and it appears that the maximum regulated concentration is to protect both the paper and the set gypsum core.
Attempts have been made to add biocides to the aqueous gypsum slurry, resulting in other problems. Water-soluble biocides tend to migrate with the excess water during the drying process, depositing on the covering when the water evaporates leaving the core unprotected. Biocides that are insoluble are difficult to disperse in the aqueous gypsum slurry and provide little to no protection for the paper facing material. Chemicals added directly to the gypsum slurry can also have detrimental effects on the properties of the set gypsum product. For example, when boric acid, a known biocide, is added to a slurry in sufficient quantity to inhibit mold growth, the panels become brittle and crack or chip as they are moved along the rollers from the kiln during normal manufacture.
Another technique of protecting a gypsum panel is using a two-step process of covering a biocide-containing gypsum slurry with a treated paper. In addition to many of the problems discussed above, use of a two-step process is more expensive than a single step process. Thus, although many biocides are well known, this particular application poses unique problems in finding a biocide that inhibits mold growth in both the paper and the core of gypsum panels in a cost effective manner.
Pyrithione salts are well-known antimicrobial additives for coating applications. They are available commercially as sodium OMADINE® or zinc OMADINE®, manufactured by Arch Chemicals, Inc. of Norwalk, Conn., or they can be made according to the process of U.S. Pat. No. 3,159,640, incorporated herein by reference. The prior art teaches that these salts are useful in their wet state as preservatives or as short-term antimicrobial agents in dry, thin-film applications such as paints, adhesives, caulks and sealants. U.S. Pat. No. 5,939,203 issued to Kappock et al., incorporated herein by reference, discloses that joint compounds and patching compounds are suitable base media for use with pyrithione salts in coating compositions. Joint or patching compounds are thinly spread over joints between or imperfections in gypsum panels, forming a thin film. The use of sodium pyrithione in these compounds acts as a wet state preservative for ready mixed products and inhibits microbial growth on the dry film of the product.
Traditional Manila papers, however, are fashioned to be aesthetically pleasing. To obtain a finish that is looks appealing, it is necessary to treat the exposed surface of the paper so that the paper is finished, can be painted, has a pleasing appearance and has a smooth surfacing, without the labor intensive requirement of re-finishing. Fly Leaf fibers are often used in papers for gypsum panels because they are short fibers that naturally form a relatively smooth surface. The paper is calendared to give it a smooth, glossy finish. Use of Kraft fibers is currently limited to papers used on the back side of gypsum panels or in plies that are not exposed to the visible surfacing.
Since Kraft fibers are typically longer, softwood fibers, when forming a paper of Kraft fibers, the surface is rough as compared to Fly Leaf fibers. Papers prepared with Kraft fibers also have a darker color than those made of Fly Leaf fiber. If paint is applied as a finish, additional coats may be required to obtain a true color if a surface size is not applied. Another problem with using Fly Leaf fibers in a visible face occurs when the surface sizing is not uniformly applied. Fly Leaf waster fibers contain more ash and are prepared by the thermal mechanical pulping process that requires more internal size. This can lead to uneven color or texture across the surface of a panel that is unappealing. As such, Kraft fiber papers typically are exposed to the wall cavity to minimize finishing and refinishing procedures.
Papers, such as those containing Fly Leaf fibers, are finished to have a pleasing appearance and smooth surfacing require additional chemicals to impart water resistant properties and retain the smaller hardwood or Fly Leaf fibers. When treated with a biocide solution, however, only about 65% to about 75% of the solution is retained by the Fly Leaf and/or News fibers. As biocides are an expensive component of the panel, cost prohibits the use of large amounts of biocides where little is retained by the paper. In addition, panels including Fly Leaf and News fibers typically have an inconsistent finish with respect to color.